/Engine/Plugins/Runtime/XRBase/Source/XRBase/Public/MotionTrackedDeviceFunctionLibrary.h
Get the number of controllers for which tracking is enabled.
(int) number of controllers tracked now, or -1 if this query is unsupported (IsMotionTrackedDeviceCountManagementNecessary()should return false).
[UFUNCTION](Programming/UnrealArchitecture/Reference/Functions)(BlueprintPure, Category="Input|MotionTracking") static int32 GetMotionTrackingEnabledControllerCount()